What are atmospheric rivers, in more scientific terms?

Three examples of AR events that produced extreme precipitation on the US West Coast, and exhibited spatial continuity with the tropical water vapor reservoir as seen in satellite observations of integrated water vapor.

The color scale used in these images represents the total amount of water vapor between the ocean surface and space.

Examples of AR events (click image for more detail).

ARs are the water-vapor rich part of the broader warm conveyor belt (e.g.,
Browning, 1990; Carlson, 1991), that is found in extratropical cyclones
("storms"). They result from the action of winds associated with the storm
drawing together moisture into a narrow region just ahead of the cold front
where low-level winds can sometimes exceed hurricane strength. The term
atmospheric river was coined in a seminal scientific paper published in
1998 by researchers Zhu and Newell at MIT (Zhu and Newell 1998). Because they found that most
of the water vapor was transported in relatively narrow regions of the
atmosphere (90% of the transport occurred typically in 4-5 long, narrow
regions roughly 400 km wide), the term atmospheric river was used. A
number of formal scientific papers have since been published building on
this concept (see the publication list),
and forecasters and climate researchers are beginning to apply the ideas
and methods to their fields. The satellite images at top-left show strong ARs
as seen by satellite. The advent of these specialized satellite
observations have revealed ARs over the oceans and have revolutionized
understanding of the global importance of ARs (more traditional satellite
data available in the past could not clearly detect AR conditions). The
interpretation of these satellite images, which represent only water vapor,
not winds, was confirmed using NOAA research aircraft data over the Eastern
Pacific Ocean and wind profilers along the coast (Ralph et al. 2004). The
event shown in the bottom-left image was documented by Ralph et al. (2006), which
concluded this AR produced roughly 10 inches of rain in 2 days and caused a
flood on the Russian River of northern California. It was also shown that
all floods on the Russian River in the 7-year period of study were
associated with AR conditions. As of late 2010 there have been a number of
papers published on major west coast storms where the presence and
importance of AR conditions have been documented. These are provided in an
informal list of the "Top Ten ARs" of the last several years on the U.S.
West Coast. It is now recognized that the well-known "Pineapple express,"
storms (a term that has been used on the U.S. West Coast for many years)
correspond to a subset of ARs, i.e., those that have a connection to the
tropics near Hawaii. In some of the most extreme ARs, the water vapor
transport is enhanced by the fact that they entrain (draw in) water vapor
directly from the tropics (e.g., Bao et al 2006, Ralph et al. 2011).

Can we forecast atmospheric rivers?

National Weather Service forecasters located along the west coast are now familiar with the concept of atmospheric rivers and can identify these phenomena in current numerical forecast models. This provides them the capability to give advanced warning of potential heavy rain sometime 5 to 7 days in advance. They have also learned to monitor polar orbiter microwave satellite imagery that provides advanced warning of the presence and movement of these phenomena in the Pacific. During the last two winters, with the development of atmospheric river observatories, forecasters have been able to monitor the strength and location of these rivers as they make landfall and thus improve short-term rainfall forecasts for flash flooding. There are still challenges to predicting rainfall totals in these events as models still struggle with the details of the duration and timing of AR's as they make landfall.

Why are ARs capable of producing extreme rainfall on the U.S. West Coast?

they are associated with strong winds that force the water vapor up mountain sides,

the atmospheric conditions do not inhibit upward motions (because the atmospheric static stability is nearly neutral up to about 3 km MSL, on average)

once the air moves upward, the water vapor condenses and can form precipitation

What is the role of atmospheric rivers in creating floods?

Research has shown there were 42 ARs that impacted CA during the winters from 1997 to 2006, and the resulting seven floods that occurred on the Russian River watershed northwest of San Francisco during this period were all associated with AR conditions.

A major flood in California, known as the "New years Day Flood" in 1997 cause over $1 Billion in damages and had a well-defined AR.

Less formally, ARs are known to result in an order of magnitude larger post-storm stream flow "bumps" (increases) than other California storms, in the Merced and American Rivers.

The Pacific Northwest also regularly experiences this type of storm. Case in point is the landfalling AR of early November 2006 that produced heavy rainfall and devastating flooding and debris flows with region-wide damage exceeding $50 million.

How are science and applications of ARs being addressed?

Research experiments (CalJet and PacJet) performed by NOAA in the 1998, 2001, and 2002 were conducted to better understand landfalling Pacific winter storms.

CalJet/PacJet led to the development of the NOAA Hydrometeorology Testbed (HMT; hmt.noaa.gov). HMT's aim is to accelerate the development and prototyping of advanced hydrometeorological observations, models, and physical process understanding, and to foster infusion of these advances into forecasting operations of the NWS, and to support the broader user community's needs for 21st Century precipitation information.

Within HMT, scientists have developed and prototyped an atmospheric river observatory (ARO) designed to further our understanding of the impact of ARs on enhancing precipitation in the coastal mountains and the high Sierra of California.

Studies of the potential impacts of climate change on AR characteristics is the focus on an ongoing project – CalWater that is partnering with HMT, the California Energy Commission, Scripps Institution of Oceanography, USGS and others, to explore the potential implications for flood risk and water supply.

Under the USGS-led Multihazards project, ARs have become the focus of an emergency preparedness scenario for California that is intended to help the region prepare for a potentially catastrophic series of ARs. The scenario is named "ARkStorm" and has developed an informational video for use with the public (http://urbanearth.gps.caltech.edu/winter-storm/).

What are the benefits of studying atmospheric rivers?

The community of flood control, water supply and reservoir operators of the
West Coast states see ARs as a key phenomenon to understand, monitor and
predict as they work to mitigate the risks of major flood events, while
maintaining adequate water supply. The frequency and strength of AR events
in a given region over the course of a typical west-coast wet season
greatly influences the fate of droughts, floods, and many key human
endeavors and ecosystems. Better coupling of climate forecasts with
seasonal weather forecasts of ARs can improve water management decisions.
Long-term monitoring using satellite measurements, offshore aircraft
reconnaissance, and land-based atmospheric river observatories, combined
with better numerical modeling, scientific progress, and the development of
AR-based smart decision aids for resource managers, will enable society to
be more resilient to storms and droughts, while protecting our critical
ecosystems.

AR Animation Loop

Animation of an atmospheric river event in February 2015. (Credit: NOAA/ESRL Physical Sciences Division)